WASHINGTON (CNN) --If President Bush makes a decision to send U.S. troops to Liberia, the most likely scenario would be for 2,000 Marines stationed in the Horn of Africa to go to the West African nation, according to U.S. military sources.

The Bush administration is considering deploying a force that would help lead troops from other West African nations in peacekeeping and humanitarian efforts in war-torn Liberia.

The most readily available U.S. troops are a Marine expeditionary unit based on the amphibious ship USS Iwo Jima, which is accompanied by the USS Nashville and USS Carter Hall.

It is unclear if all three ships would be sent. These Marines are near the East African nation of Djibouti, where they plan to conduct exercises as they return from duty in the Persian Gulf.

But it could take more than two weeks to get those Marines to West Africa. No Marine expeditionary units are ready for deployment on the U.S. East Coast, however, so planners may still choose to use the Iwo Jima.

Pentagon planners said the advantage of using the Marines would be U.S. ships offshore as a flexible base of operations. It would allow them to use combat helicopters to ferry troops and supplies back and forth to the Liberian capital of Monrovia as well as to provide a logistics and supply point, including medical care. If Army troops were sent, they would have to be accompanied by support and supply units.

One planning scenario calls for the Marines to stay no longer than three months, U.S. military sources said. Options under review call for 30-, 60- and 90-day deployments.

Military planners said they want to see an agreed-upon and achievable goal for the operation so that there can be a clear understanding of when troops can come home.

Pentagon planners also are wondering if U.S. troops would land in Liberia unilaterally first or go in conjunction with West African military forces, military sources said. Another Pentagon option would be to establish a timetable for turning over the U.S. mission to West African troops.

A U.S. humanitarian assessment team in Liberia is expected to determine what specialists or peacekeeping forces would be needed to create conditions to encourage international relief workers to return to the nation in large numbers.